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Batch size (BATCHSZ)

 

The batch size is the maximum number of messages to be sent before a syncpoint is taken. The batch size does not affect the way the channel transfers messages; messages are always transferred individually, but are committed or backed out as a batch.

To improve performance, we can set a batch size to define the maximum number of messages to be transferred between two syncpoints. The batch size to be used is negotiated when a channel starts up, and the lower of the two channel definitions is taken. On some implementations, the batch size is calculated from the lowest of the two channel definitions and the two queue manager MAXUMSGS values. The actual size of a batch can be less than this; for example, a batch completes when there are no messages left on the transmission queue or the batch interval expires.

A large value for the batch size increases throughput, but recovery times are increased because there are more messages to back out and re-send. The default BATCHSZ is 50, and you are advised to try that value first. You might choose a lower value for BATCHSZ if your communications are unreliable, making the need to recover more likely.

Syncpoint procedure needs a unique logical unit of work identifier to be exchanged across the link every time a syncpoint is taken, to coordinate batch commit procedures.

If the synchronized batch commit procedure is interrupted, an in-doubt situation may arise. In-doubt situations are resolved automatically when a message channel starts up. If this resolution is not successful, manual intervention may be necessary, making use of the RESOLVE command.

Some considerations when choosing the number for batch size:

• If the number is too large, the amount of queue space taken up on both ends of the link becomes excessive. Messages take up queue space when they are not committed, and cannot be removed from queues until they are committed.

• If there is likely to be a steady flow of messages, we can improve the performance of a channel by increasing the batch size. However, this has the negative effect of increasing restart times, and very large batches may also affect performance.

• If message flow characteristics indicate that messages arrive intermittently, a batch size of 1 with a relatively large disconnect time interval may provide a better performance.

• The number may be in the range 1 through 9999. However, for data integrity reasons, channels connecting to any of the current platforms, as described in this book, should specify a batch size greater than 1. (A value of 1 is for use with V1 products, apart from MQSeries for MVS/ESA™.)

• Even though nonpersistent messages on a fast channel do not wait for a syncpoint, they do contribute to the batch-size count.

This attribute is valid for channel types of:

• Sender

• Server

• Receiver

• Requester

• Cluster sender

• Cluster receiver

 

Parent topic:

Channel attributes in alphabetical order

ic11560_

 

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